I have an old 128GB SSD that i use as an external drive in a eSATA case.

When I copy a lot of data on it, after some time, the transfer rate drops below 1 MBps, just like if the cache would be full.If i launch a test using the disk program in Gnome3, the transfert rate is 200 MBps read / 120MBps write, even with a hundred 1000MB samples.